Reduced Ψ risks cavitating xylem tissue, thereby lowering plant hydraulic conductance ( Tyree and Sperry, 1989), while reduced photosynthesis potentially lowers growth rates. More specifically, water stress can reduce water potential (Ψ), stomatal conductance (g s), and photosynthetic rates in seedlings ( Ritchie and Shula, 1984 Warren et al., 2004). Following drought-induced tree mortality in a climate predicted to continue warming and drying for decades ( Allen et al., 2010, 2015), the successful establishment and regeneration of former forestlands is dubious, as water stress can threaten successful seedling establishment in Mediterranean climates with hot, dry summers ( Plamboeck et al., 2008). Droughts are therefore a major threat to forest health and perpetuation in the West, as water stress can reduce tree hydraulic function via excessive xylem cavitation and can reduce tree carbon uptake via excessive stomatal regulation to conserve water ( McDowell et al., 2008). As forests are an important national renewable resource that supply valuable timber products and support diverse economies, this loss of resources reduces timber production and carbon sequestration and also increases the risk of catastrophic wildland fires ( Jolly et al., 2015). In parts of the central and southern Sierra Nevada mountains of California, epidemic tree mortality has exceeded 50% for some species ( Asner et al., 2016 Byer and Jin, 2017 Young et al., 2017 Stephenson et al., 2019), with millions of trees having died in response to the recent 2012 – 2015 drought ( Plamboeck et al., 2008 Fettig et al., 2019). Forest management to foster productive, healthy, resistant, and resilient forests in the face of increasing drought frequency and severity is an important and widespread goal across many land ownerships ( Keenan, 2015 Newton and Preest, 1988 Millar and Stephenson, 2015).ĭrought-induced forest mortality and subsequent regeneration patterns are major concerns in the western United States. Many regions can expect warming of extreme temperatures, some regions can expect increases in drought intensity or frequency, and drought-prone regions can expect more severe droughts ( Jia et al., 2019). The forecasted increase in drier climates worldwide, with increasing exposure to temperature and rainfall extremes, has the potential to alter forest structure and function ( Allen et al., 2010). Global climate change is forecast to have widespread impacts on forested ecosystems ( Jia et al., 2019). Our results also demonstrate that redwood, a species generally restricted to mesic coastal habitats, can successfully establish in xeric inland sites when planted after partial harvesting, even during drought conditions.
Overall, our study indicates that low-retention silvicultural treatments can minimize water stress and maximize gas exchange, growth, and survival in regenerating seedlings. We also found that compared to Douglas-fir, redwood had higher Ψ and g s, greater growth, and lower mortality. Compared to seedlings in the no-cut and moderate-retention treatments, seedlings in the gap (100% cut) treatment had the highest Ψ and g s, greatest growth, and lowest mortality. To study the influences of stand structure and climate on regeneration success, we monitored physiology, growth (change in basal diameter and biomass accumulation), and mortality rate of planted Douglas-fir ( Pseudotsuga menziesii) and coast redwood ( Sequoia sempervirens) seedlings for two growing seasons after partial harvesting in inland northern California.
Moreover, as climate throughout the West is projected to become hotter and drier, it is important to investigate regeneration under xeric conditions, particularly for species restricted to mesic habitats.
As disturbances such as drought-induced mortality and wildland fires spread across many forests of the western United States, a better understanding of the influences of stand structure on seedling physiology can foster more effective reforestation efforts.
Reforestation following timber harvests and natural disturbances is an essential component of sustainable forest management. 3Department of Wildlife, Humboldt State University, Arcata, CA, United States.2Department of Biological Sciences, Humboldt State University, Arcata, CA, United States.1Department of Forestry and Wildland Resources, Humboldt State University, Arcata, CA, United States.Lucy Kerhoulas 1*, Wade Polda 2, Nicholas Kerhoulas 3 and John-Pascal Berrill 1
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